Method of forming rigid layer on titanium and titanium alloy having rigid layer formed by the same

ABSTRACT

Disclosed is a method capable of inexpensively forming a gradient-hardened rigid layer which has characteristics of functionally graded material on the surface layer of titanium. The method includes (a) injecting titanium into a heat treatment apparatus and performing ventilation to maintain an atmospheric pressure of 10 −4  torr or less, (b) performing a pretreatment process of heating the titanium at 730 to 800° C. for 10 minutes to 5 hours to remove an oxide film formed on the surface of the titanium, (c) injecting one or more gases selected from nitrogen, oxygen, and carbon into the heat treatment apparatus and heating the titanium at 740 to 950° C. for 30 minutes to 20 hours such that a gradient-hardened rigid layer having a concentration gradient of the gases is formed on the surface of the titanium, and (d) cooling the titanium.

TECHNICAL FIELD

The present invention relates to a method of forming a continuouslygradient-hardened rigid layer on the surface layer of titanium (puretitanium and titanium alloy), and more particularly, to a method offorming a rigid layer having characteristics in which physicalproperties are continuously varied at low costs on the surface layer oftitanium and titanium on which the rigid layer is formed by the method.

BACKGROUND ART

Pure titanium and titanium alloy have widely been applied to militaryand aerospace industrial fields in which weight lightening is essentialdue to their relative light weight compared to other structuralmaterials and high specific strength in a temperature range fromextremely low temperatures to high temperatures of 400 to 500° C.Further, titanium has mostly been used as biomaterial inserted into thehuman body due to its superior corrosion resistance andbiocompatibility. Additionally, titanium has recently been expanding itsapplications even in civilian articles requiring revelation of variousshades since anodizing enables coloring of titanium.

Studies on titanium surface modification for improving titaniumproperties has recently been receiving attention since titanium limitsits application fields due to its poor surface hardness and wearresistance in spite of such excellent properties of titanium.

In this connection, a method of forming a rigid film such as TiN on thesurface of a titanium has been suggested as a method for improvingsurface properties, particularly, hardness and wear resistance of thetitanium.

A TiN film among wear resistance-improving film materials, which hasactively been studied, is a film material that has widely been used forcorrosion resistance or decorating as well as wear resistance since theTiN film is excellent in oxidation resistance, has superior surfaceroughness and ductility, and is beautiful due to its yellow color.

However, when TiO₂ is formed through the oxidation process, the TiN filmis subjected to very large volume expansion with a volume expansionratio of about 64% to result in the formation of a large compressivestress in the formed oxide layer. Therefore, the TiN film has a drawbackthat cracks in the film are caused to rapidly proceed oxidation in ahigh temperature atmosphere of 500° C. or higher.

As a method that is capable of overcoming such a drawback of the TiN, anoxynitride material including oxygen such as TiN_(x)O_(y) may be used asa film for surface hardening since an oxynitride film material based ona ternary system also has excellent hardness, electrical properties,wear resistance and corrosion resistance as in the TiN film by chemicalbonds of atoms within lattices and electrical structures of oxynitrides.

On the other hand, examples of a method of forming a titanium oxynitridefilm having excellent properties such as TiN_(x)O_(y) on the surface ofa titanium alloy may include nitriding, carburizing, thermal sprayprocess, physical vapor deposition (PVD), and chemical vapor deposition(CVD).

Among the examples of the method, the PDV method such as ion plating,cathode arc deposition and reactive sputtering, or the CVD method usingplasma is mainly considered.

Although the PVD process has merits that deposition is performed atlower temperatures than the CVD process, structural changes areminimized in the interface between the coating layer and the surface ofa titanium alloy, and a coating layer with excellent wear resistance,heat resistance, oxidation resistance and corrosion resistance can beformed, there are demerits in that there is weak adhesive strengthbetween the coating layer and a titanium alloy matrix, a coatingapparatus is expensive, and it takes a long time to form the coatinglayer.

Further, although the CVD process has a merit of facilitating thecomposition of the coating layer and control of coating thickness, thereis a demerit in the CVD process that the deposition process mainlyoccurs at high temperatures such that the structural changes are causedin the interface between the coating layer and the titanium alloy toresult in a bad effect exerted on mechanical properties and corrosion ofthe titanium alloy accordingly.

On the other hand, there are problems that separation and cracking ofthe film occur under the conditions of an external impact andmulti-axial loading since a double layer is formed which is separatedinto a titanium alloy matrix layer with a relatively low hardness and acoating layer with a relatively high hardness when coating a hard filmon the surface of a titanium alloy by all of the above-mentionedover-layer coating methods, wherein the matrix has metal characteristicswhile the coating layer has ceramic characteristics.

Further, when performing thermal spray coating, it is hard to expectphysical properties such as sufficient oxidation resistance since lotsof pores are contained in the coating layer, and it is also difficult toform a compact coating layer since the coating layer basically possessessuch defects as pores although less pores are formed in a coating layerformed by the PVD or CVD process. Furthermore, since over-layer coatingincludes adding the coating layer to processed parts, there are problemsthat there is a growing need to perform the post-process to result in anincrease in the manufacturing costs of the parts accordingly due tosevere dimensional changes of the coating layer undergone between beforethe coating process and after the coating process in case of forming athick coating layer.

Further, there has been a problem that physical properties of the matrixstructure deteriorate or treatment costs increase since changes in thetitanium alloy matrix structure are caused by a long time consumed toobtain predetermined physical properties in case of an inner-layercoating method such as carburizing or nitriding.

DISCLOSURE OF THE INVENTION Technical Problem

The present invention is obtained through researches and development tosolve the defects of the foregoing prior art, and provides a method offorming a rigid layer of titanium, which is capable of obtaining therigid layer with excellent physical properties at low costs byperforming a Thermo-Chemical Treatment (TCT) of diffusing andpenetrating interstitial elements such as oxygen, carbon and nitrogenfrom the surface of titanium, thereby simultaneously forming aninner-layer coating layer to solve the defects of an over-layer coatinglayer and conducting a continuous treatment of one time for a shortperiod of time without performing a separate process of removing theoxide film on the surface of titanium or suppressing the formation of anoxide film on the titanium surface when forming the inner-layer coatinglayer.

Furthermore, another aspect of the present invention is to providetitanium from the surface of which a rigid layer having a concentrationgradient is formed using interstitial elements such as oxygen, carbon ornitrogen. The rigid layer is obtained by the above-mentioned method.

Technical Solution

In order to accomplish the foregoing objects, the present inventionprovides a method including the steps of (a) injecting titanium into avacuum heat treatment apparatus and performing ventilation to maintainan atmospheric pressure of 10⁻⁴ torr or less, (b) performing apretreatment process of heating the titanium at 730 to 800° C. for 10minutes to 5 hours to remove an oxide film formed on the surface of thetitanium and its alloy, (c) performing a hardening process of injectingone or more gases selected from nitrogen, oxygen, and carbon into thevacuum heat treatment apparatus and heating the titanium at 740 to 950°C. for 30 minutes to 20 hours such that a continuously gradient-hardenedrigid layer having a concentration gradient of the gases is formed onthe titanium surface, and (d) cooling the titanium and its alloy.

In a method according to the present invention, the atmospheric pressureof the step (a) may be 5×10⁻⁵ torr or less.

Further, in a method according to the present invention, the step (b)may be performed at 740 to 780° C.

Further, in a method according to the present invention, the step (b)may be performed for 10 minutes to 1 hour.

Further, in a method according to the present invention, the step (c)may have a temperature higher than that of the step (b).

Further, in a method according to the present invention, the step (c)may be performed at 740 to 850° C.

Further, in a method according to the present invention, the step (c)may be performed for 30 minutes to 5 hours.

Further, in a method according to the present invention, the step (d)may be performed by cooling titanium by a step cooling method.

Further, in a method according to the present invention, the step (d)may include an aging step of maintaining titanium at 500 to 800° C. for30 minutes to 30 hours.

Further, a method according to the present invention may include, afterthe step (d), a step of additionally forming a coating layer for thepurpose of fingerprinting resisting and color manifestation on thesurface of titanium using over-layer coating methods such as a CVDmethod and a PVD method, etc.

Further, in a method according to the present invention, the titaniummay be pure titanium or a titanium alloy.

Furthermore, the present invention provides titanium having a rigidlayer formed by the above-mentioned method, and various parts utilizingits alloys and technologies thereof.

Advantageous Effects

The following effects may be obtained by the present invention.

First, a method according to the present invention is not onlyeconomical, but also advantageous in blocking structural changes of amatrix due to gas permeation since the method is capable of obtainingphysical properties of the same level or higher within a short period oftime compared to a convention process of permeating interstitial gases.

Second, a rigid layer having very excellent physical properties can beobtained by forming a thick rigid layer on the surface of a titaniumalloy and obtaining the control effect of a matrix structure through thestep cooling method that is an embodiment of the present invention.

Third, a method according to the present invention is capable ofsimplifying the process and obtaining an excellent inclined rigid layerat low cost by performing removal of an oxide film on the surface of thetitanium alloy and formation of an inclined coating layer en bloc in anapparatus.

Fourth, a delamination phenomenon does not occur in the interface sincea concentration gradient of interstitial elements is formed in the basemetal direction from the surface of the rigid layer formed according tothe present invention such that physical properties are continuouslyvaried between the rigid layer and the base metal.

Fifth, the rigid layer formed according to the present invention iseconomical since the interstitial elements are injected into the basemetal, and there are hardly any dimensional changes in the rigid layeraccordingly such that a subsequent forming treatment is not required tobe performed after performing a hardening treatment.

Sixth, defects of a thermal spray coating layer, a PVD coating layer, ora CVD coating layer do not occur since the interstitial elements areinjected into a crystal lattice of the base metal in the rigid layerformed according to the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a mimetic diagram of a hardening layer forming process accordingto Example 1 of the present invention.

FIG. 2 a mimetic diagram of a hardening layer forming process accordingto Example 2 of the present invention.

FIG. 3 illustrates photographs of an optical microscope for observingsurfaces of rigid layers formed according to Examples 1 and 2 of thepresent invention and Comparative Example.

FIG. 4 illustrates photographs of a scanning electron microscope forobserving cross-sectional views of rigid layers and PVD coating layersformed according to Examples 1 and 2 of the present invention andComparative Example.

FIG. 5 illustrates measurement results of the surface roughness of rigidlayers and PVD coating layers formed according to Examples 1 and 2 ofthe present invention and Comparative Example.

FIG. 6 illustrates measurement results of the friction characteristicsof rigid layers formed according to Examples 1 and 2 of the presentinvention and Comparative Example.

FIG. 7 illustrates measurement results of the cross-sectional hardnessof rigid layers formed according to Examples 1 and 2 of the presentinvention and Comparative Example.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, although the present invention is described in detail basedon preferred examples of the present invention, the present invention isnot limited to the following examples.

The present inventors have paid attention to the inner-layer coatingmethod such as carburizing or nitriding after taking notice of problemsthat it is fundamentally difficult to prevent a rigid film from beingdelaminated due to an external force, and dimensional change due toover-layer coating generates inevitable subsequent processing since adouble layer in which differences in physical properties arefundamentally remarkable in the interface between a titanium alloymatrix and a coating layer when coating is performed on the surface ofpure titanium and a titanium alloy by over-layer coating methods such asthermal spray coating, PVD, and CVD, etc.

It is necessary to remove a stable titanium oxide layer formed on thesurface of titanium or titanium alloy in order to promptly permeate aninterstitial element such as oxygen, nitrogen or carbon used in theinner-layer coating method. In related arts, a method has been used thatremoves the oxide film on the titanium surface and carries out surfacehardening by performing direct surface treatment without a separateprocess to remove the oxide film or by performing a plasma nitridingmethod of ionizing some of nitrogen gas through glow discharge in a lowpressure nitrogen atmosphere, thereby colliding the ionized nitrogen gaswith the titanium surface. However, these methods have limitations thatprocess treatment temperatures are high, and surface treatment of aparticularly long period of time should be carried out to obtain ahardening layer with desired physical properties. In another words,methods of the related art not only require a long period of time, butalso would be completed in a single process to result in a considerablyhigh amount of cost consumed in the formation of the hardening layer.

In order to overcome such problems, the present inventors have studieson diverse inner-layer coating methods. As a result of the study, thepresent invention has been accomplished by maintaining a gas pressure ina heat-treatable vacuum chamber to a high degree of vacuum of 10⁻⁴ torror high within a specific temperature range such that a several angstromto nanometer-thick oxide film formed on the surface of titanium may beremoved for a short period of time, and subsequently maintaining thetemperature range and the gas pressure such that the interstitialelement such as nitrogen, oxygen, or carbon is easily permeated intotitanium to confirm that a rigid layer of excellent physical propertieswith an inclination function is obtained even in a short period of time.A rigid layer formed according to the present invention is capable ofpreventing separation or cracking between the coating layer and thematrix surface due to external effects, that is occurred in existingover-layer coating, and is capable of continuously performing surfacehardening without separately performing processes of removing the oxidefilm and suppressing the formation of the oxide film particularly usinga multistep heat diffusion surface hardening process technology suchthat surface hardening can be performed that not only has optimalphysical properties, but also is economically beneficial.

A method of forming a titanium rigid layer according to the presentinvention includes the steps of: (a) injecting titanium into a vacuumheat treatment apparatus and performing ventilation to maintain anatmospheric pressure of 10⁻⁴ torr or less, (b) performing a pretreatmentprocess of heating the titanium at 730 to 800° C. for 10 minutes to 5hours to remove an oxide film formed on the surface of the titanium, (c)injecting one or more gases selected from nitrogen, oxygen, and carboninto the vacuum heat treatment apparatus after the removal of the oxidefilm and heating the titanium at 740 to 950° C. for 30 minutes to 20hours such that a rigid layer is formed on the titanium surface, and (d)cooling the titanium.

In the present invention, “titanium” is used as a meaning including puretitanium and a titanium alloy.

The heat treatment apparatus should maintain an atmospheric pressure of1×10⁴ torr or less as a degree of vacuum since the oxide film formed onthe surface of the titanium cannot be removed clearly in the step (b) incase of the atmospheric pressure of 1×10⁻⁴ torr or less to result in aninsufficient effect of the subsequent hardening process. Therefore, itis more preferable to maintain the atmospheric pressure of the heattreatment apparatus to an atmospheric pressure of 5×10⁻⁵ torr or less.

The step of removing the oxide film should be conducted at a temperatureof 730 to 800° C. since the oxide film is not sufficiently removed toresult in an increase of the subsequent permeation process of theinterstitial gas when the temperature is less than 730° C., and thereare disadvantages in terms of microstructural and mechanical propertieswhen the temperature is more than 800° C. More preferably, the step ofremoving the oxide film should be conducted at a temperature range of740 to 780° C. Further, it is more preferable that the step of removingthe oxide film should be conducted when an oxide film-removing time isfrom 10 minutes to 1 hour since removal of the oxide film is notcompletely accomplished when the oxide film-removing time is less than10 minutes, and there are disadvantages in terms of an economical aspectand mechanical properties when the oxide film-removing time is more than1 hour.

The hardening step may be continuously conducted together with removalof the oxide film, or may be carried out in a second-step heat treatmentmethod by increasing the temperature to higher than the oxidefilm-removing temperature, wherein the second-step heat treatment methodis more preferable since hardening time is shortened. Namely, surfacehardening time may considerably shortened by employing the pre-treatmentprocess before conducting hardening heat treatment by the interstitialgas element.

The hardening step may be conducted at a temperature of 740° C. orhigher, and may be conducted at a temperature up to 850° C. according todepth and required hardness of the hardening layer. More preferably, thehardening step is conducted at a temperature of not less than 780° C.which is higher than the oxide film-removing temperature. Further, thehardening time is preferably from 30 minutes to 20 hours since thepermeation amount of the interstitial element such as oxygen, nitrogenor carbon is insufficient in case of the hardening time of less than 30minutes, and there are disadvantages in terms of an economical costaspect and a drop in mechanical properties according to microstructuralgrowth of grain size in case of the hardening time of more than 20hours.

Further, the gas used in the hardening step may include carbon,nitrogen, oxygen, and mixture gases thereof elements that may easilypermeate into lattices of a titanium crystal.

The cooling step may be performed by a furnace cooling method or an aircooling method in a heat treatment furnace, and a step cooling methodincluding an aging step of maintaining titanium at 500 to 800° C. for 30minutes to 30 hours to uniformize structure of a rigid layer formed andform a thick rigid layer on a surface part of the rigid layer may beused during furnace cooling or fast cooling.

Examples 1

A commercial pure titanium (Gr. 2 material) having a size of 30 mm(length)×25 mm (width)×2 mm (height) as a surface hardening sample usedin the Example 1 of the present invention was used, and a chemicalcomposition of a Gr. 2 material suggested by the manufacturer isrepresented in the following Table 1.

TABLE 1 Chemical components C Fe H N O Ti Content (weight %) 0.05 0.30.008 0.02 0.2 Balance

After immersing a prepared titanium sample into an acetone solution tosubject the titanium sample to ultrasonic cleaning and drying theultrasonic-cleaned titanium sample, the dried ultrasonic-cleanedtitanium sample was subjected to surface hardening by a method such as amimetic diagram as illustrated in FIG. 1.

Specifically, after charging the sample into the chamber of a GasControlled Vacuum Furnace (GCVF, the chamber was decompressed to apressure of 5×10⁻⁶ torr using a vacuum pump. Subsequently, afterincreasing temperature of a heat treatment furnace to 750° C., the heattreatment furnace with the increased temperature was maintained for 30minutes such that a titanium oxide film with a thickness of about 10 Ånaturally formed on the surface of the titanium sample was removed bythermal decomposition.

After removing the oxide film, 100 ccm of a mixture gas of oxygen andnitrogen was injected into the chamber, and partial pressure within thechamber was adjusted such that a pressure within the chamber wasmaintained to 5×10⁻¹ torr. After increasing temperature of the heattreatment furnace to 800° C., and the heat treatment furnace with theincreased temperature was maintained for 3 hours such that the injectedoxygen and nitrogen elements could permeate into the titanium samplefrom the surface of the titanium sample. After completing hardening, thetitanium sample was cooled by an fast cooling method or a furnacecooling method.

Further, the titanium sample had a TiN coating layer formed thereon bythe PVD method in order to lower surface roughness of the titaniumsample and realize uniform colors of the titanium sample. A PVD coatinglayer was formed at 150° C. for 10 minutes in a nitrogen gas atmosphereby using a Ti target, and the formed PVD coating layer had a thicknessof 2.1 μm.

Example 2

A rigid layer was formed in the Example 2 of the present invention by amethod, i.e., a step cooling method including conducting pre-treatmentand hardening of the same sample as that in the Example 1 of the presentinvention under the same conditions as those in the Example 1 of thepresent invention, performing an aging process of maintaining the samplein a heat treatment furnace at 700° C. for 1 hour in order to homogenizemicrostructure of the sample, maximize strength of the sample, andincreasing thickness of the surface rigid layer of the sample in thecooling step, and then taking out the sample from the heat treatmentfurnace to cool the sample by air cooling as illustrated FIG. 2. Thetitanium sample had a TiN coating layer formed thereon by the PVD methodin order to lower surface roughness of the titanium sample and realizeuniform colors of the titanium sample in the same method as that in theExample 1. A PVD coating layer was formed at 150° C. for 10 minutes in anitrogen gas atmosphere by using a Ti target in the same method as thatin the Example 1, and the formed PVD coating layer had a thickness of2.1 μm.

Comparative Example

In the Comparative Example, a titanium sample prepared in the samemethod as in the Example 1 had a TiN coating layer formed on the surfacethereof by a PVD method. The TiN coating layer was formed at 150° C. for10 minutes in a nitrogen gas atmosphere by using a Ti target, and theformed TiN coating layer had a thickness of 3.4 μm.

Surface shape, cross-sectional shape, cross-sectional hardness, surfaceroughness, surface wear characteristics, and the like of the surfacerigid layer formed as described above were analyzed.

Specifically, surface shape was observed through an optical microscope,and the cross-sectional shape was observed through a scanning electronmicroscope. Further, the surface roughness was measured in the statethat scan length was fixed to 9,000 μm by using a surface profiler(Model TENCOR P-11). Further, wear properties were measured by aball-on-disk type abrasion tester (Model JLTB-02 tribometer manufacturedby J&L Corporation), wherein stainless steel balls with a diameter of 1mm were used as counter material, and balls and samples were subjectedto abrasion friction under conditions of a radius of rotation of 3 mm, arotation velocity of 100 rpm, and a load of 1 N to measure frictioncoefficients of the respective samples and observe friction behaviors ofthe samples. Further, cross-sectional hardness of a rigid layer wasmeasured after cutting the samples into the inclined plane and polishingthe cut samples in order to effectively measure cross-sectional hardnessof thin plate-like samples. Hardness values of the samples were measuredfrom the surface of the samples to the central part of matrix whilemaintaining a load of 100 g for 10 seconds using a Micro-VickersHardness Tester (Model FM-700 manufactured by Future-Tech Corporation).

Surface and Cross-Sectional Structure

FIG. 3 illustrates photographs for observing the surface of a titaniumsample on which a rigid layer is formed by an optical microscopeaccording to the Examples 1 and 2 of the present invention andComparative Example.

FIG. 3 illustrates results in which surface shapes of the samples weremagnified to 50 times and 200 times respectively using an opticalmicroscope, wherein apparent structures of the respective samples allshowed surface structures having an equiaxed shaped a phase.

Further, cross-sectional shapes of the three surface hardened sampleswere observed using a scanning electron microscope. The observeddiffusion rigid layers were illustrated in FIG. 4 by observing diffusionrigid layers of the samples after polishing cross-sections of thesamples into an inclined plane in order to observe a formed thin filmlayer, a boundary between the thin film layer and matrix, and adiffusion layer to a wider range. As illustrated in FIG. 4, coatinglayer boundary surfaces were all confirmed from the three samples.

Specifically, it could be known in case of FIG. 4 a (ComparativeExample) that a surface rigid layer with a thickness of about 3.4 μm wasformed by PVD. Further, it is illustrated that the boundary betweenmatrix and thin film layer is the clearest by performing PVD only on thesurface of pure titanium without pretreatment. Accordingly, surfacefriction properties are also dropped most as illustrated in FIG. 6A.This comes from that a thin film separation phenomenon is represented ina test of a severe environment such as abrasion test according asadhesive strength between the matrix and coating layer is low.

In case of FIG. 4B (Example 1), there is a thick surface rigid layerhaving a total surface rigid layer of about 84 μm including a TiN thinfilm layer with a thickness of about 2.1 μm formed by PVD.

Furthermore, in case of FIG. 4C (Example 2), it can be seen that thereis the thickest surface rigid layer with a thickness of about 99 μmincluding a TiN thin film layer with a thickness of about 2.1 μm formedby PVD and a rigid layer formed by the TCT process. Further, aging isadditionally conducted to prevent a thin film separation phenomenonduring abrasion test by promoting diffusion of interstitial elements,thereby inclinedly changing the boundary of the diffusion layer. Due tosuch a reason, the sample according to Example of the present inventionillustrates the best surface friction properties as described belowreferring to FIG. 6C.

Surface Roughness and Surface Friction Properties

FIG. 5 shows results represented as arithmetic mean roughness values(Ra) by measuring surface roughness values to investigate the formingstate of a rigid layer formed according to Examples 1 and 2 of thepresent invention and Comparative Example, effect of the respectiveprocess conditions on the surface, and directional properties of therigid layer.

As confirmed in FIG. 5, the rigid layer formed according to ComparativeExample has an arithmetic mean roughness value (Ra) of 0.13 μm, therigid layer formed according to Example 1 has an arithmetic meanroughness value (Ra) of 0.13 μm, and the rigid layer formed according toExample 2 has an arithmetic mean roughness value (Ra) of 0.14 μm.

That is, it can be seen that surface roughness values in case of forminga PVD coating layer on the surface of rigid layers formed according toExamples 1 and 2 of the present invention are similar to those in caseof performing no surface hardening without large changes, and it can beseen that results of the surface roughness values are almost similar tothose in Comparative Example in which PVD is directly formed on thetitanium matrix. That is, it may be seen that no great effect on thesurface roughness of a final PVD coating layer is obtainable althoughhardening or aging after hardening of the titanium matrix is conducted.

Surface friction properties were obtained by conducting a wear testusing stainless steel balls as counter material under non-lubricant inthe atmosphere, and abrasion test was measured up to a number ofalternating motions of 20,000 considering initial abrasion.

FIG. 6 illustrates surface friction coefficients measured by abrasiontest, wherein FIG. 6A is an abrasion test result of a sample on whichhardening layer is formed by Comparative Example, FIG. 6B is an abrasiontest result of a sample on which hardening layer is formed by Example 1,and FIG. 6C is an abrasion test result of a sample on which hardeninglayer is formed by Example 2.

As confirmed in FIG. 6A, Comparative Example shows the highest frictioncoefficient value by representing an average friction coefficient μ of0.53. Contrary to this, a sample according to Example 1 of the presentinvention represents an average friction coefficient μ of 0.42, and asample according to Example 2 of the present invention represents anaverage friction coefficient μ of 0.44. Therefore, the average frictioncoefficients of the samples according to Examples 1 and 2 wereremarkably lower than the average friction coefficient of the sampleaccording to Comparative Example.

Such a result is evaluated to be attributed to matrix reinforcement byhardening, high hardness of a formed rigid layer itself, and superioradhesive strength between a matrix and a thin film due to a heatdiffusion method according to Examples of the present invention comparedto a coating layer formed only by PVD.

Particularly, friction coefficient values of the three samples showclear differences in the initial variations of turnover number of nomore than about 3,000 cycles, wherein the sample according toComparative Example represents a rapidly increasing pattern in which thefriction coefficient value of the sample according to ComparativeExample reaches the average friction coefficient value before theturnover number of 50 cycles differently from the samples according toExamples 1 and 2.

In comparison, the sample according to Example 1 represents superiorwear resistance properties than the sample according to ComparativeExample by reaching the average friction coefficient value at theturnover number of about 500 cycles, and particularly the sampleaccording to Example 2 represented the most excellent wear resistanceproperties by maintaining the lowest friction coefficient value to theturnover number of about 2,500 cycles.

Due to a clear property difference (i.e., hardness difference) between amatrix and a PVD TiN layer formed by Comparative Example, the TiN layeris easily separated and come away from the matrix in the early stage ofabrasion test to result in an initial rapid increase in the frictioncoefficient.

In comparison, thin films are not separated since the thin films havebeen reinforced with functionally graded material in which a clearboundary line does not exist between the matrix and the rigid layerthrough the TCT process in case of the samples according to Examples 1and 2 of the present invention. On the other hand, the sample accordingto Example 2 seems to reach the average friction coefficient value moststably and slowly since toughness of the matrix is increased, andstrength of the matrix is improved through the recovery process due toaging.

On the other hand, a matrix CP Ti (Gr. 2) for three samples used foranalyzing wear properties had an average friction coefficient value ofabout 0.7, all of the three samples representing relatively low frictioncoefficient values compared to a non-treated specimen. It is judged thatthe coating layer is not completely removed up to the turnover number of20,000 cycles due to a small load of 1 N, and a certain effect ofincreasing surface hardness through physical deposition only can beobtained. However, it can be seen that further improved surface wearresistance properties can be obtained by the TCT process of single ormultiple treatment of aging.

Cross-Sectional Hardness

Surface hardness values of three samples were measured to study if anitride was formed on the surface of a pure titanium matrix and effectsof surface treatment conditions of respective samples on the formationof the nitride. Hardness values of the three samples according toComparative Example and Examples 1 and 2 were measured as 373 Hv, 441Hv, and 489 Hv respectively while Vicker's hardness of pure titanium CPTi (Gr. 2) on which surface treatment had not been conducted weremeasured as about 167 Hv. Namely, it can be seen that surface hardnessvalues of the samples increased considerably by the formation of rigidfilms regardless of a surface hardening method.

However, it can be seen that hardness values of the samples according toExamples 1 and 2 in which a matrix is reinforced by the TCT process toconduct thin film coating on the reinforced matrix are more improvedthan hardness value of a sample on which a simple PVD TiN layer isformed by directly coating a TiN thin film on a CP Ti matrix. This showsthat the heat treatment processes according to Examples 1 and 2 areeffective in obtaining stronger surface strength.

FIG. 7 illustrates measurement results of hardness changes along thecross-sectional depths from surfaces of the samples according toComparative Example and Examples 1 and 2 using Vicker's hardness tester.

The cross-sectional hardness is a result of measuring hardness from thesurface of a sample to the central part of a matrix. The outermostsurface parts of the samples represented the cross-sectional hardnessvalues that are at least three times higher than that of the matrix byshowing that cross-sectional hardness values of the samples according toComparative Example and Examples 1 and 2 were about 340 Hv, about 450Hv, and about 500 Hv or more respectively.

Further, it is observed that a rigid layer of the sample according toComparative Example has a thickness of within several micrometers andhas its hardness dropped discontinuously and rapidly to about 160 Hv ofthe standard hardness value of CP Ti formed after the surface rigidlayer.

Comparably, hardness values of the samples according to Examples 1 and 2of the present invention were continuously decreased according toincreases in depths from surfaces of the sample, wherein the sampleaccording to Example 1 maintained a hardness value higher than that ofthe matrix up to a depth of about 80 μm, and the sample according toExample 2 maintained a hardness value higher than that of the matrix upto a depth of about 100 μm. Such results mean that interstitial elementssuch as nitrogen and oxygen are diffused into the matrix up torespective converged predetermined depths to form rigid layers, andthese results correspond with observation results of cross-sectionalstructures of the samples by a scanning electron microscope.

Further, hardness distribution results of such functionally graded rigidlayers correspond with trends that, compared with friction propertiespreviously evaluated and analyzed as illustrated in FIG. 6, a frictioncoefficient value of the sample according to Example 2 is most slowlyrisen in spite of an initial increase in friction cycles due to a highinternal hardness value and a more thickly formed functionally gradedrigid layer such that it reaches an average friction coefficient forother parts except the rigid layer and is converged at last when thefriction cycles reach 2,500 cycles or more.

It can be seen from above-mentioned results of the friction propertiesand hardness properties that a method of forming a rigid layer of thepresent invention enables a rigid layer having excellent physicalproperties to be formed within a short time compared to a conventionalmethod of forming a rigid layer.

1. A method of forming a gradient-hardened rigid layer on a surface oftitanium, the method comprising the steps of: (a) injecting titaniuminto a heat treatment apparatus and performing ventilation to maintainan atmospheric pressure of 10⁻⁴ torr or less; (b) performing apretreatment process of heating the titanium at 730 to 800° C. for 10minutes to 5 hours to remove an oxide film formed on the surface of thetitanium; (c) injecting one or more gases selected from nitrogen,oxygen, and carbon into the heat treatment apparatus and heating thetitanium at 740 to 950° C. for 30 minutes to 20 hours such that agradient-hardened layer having a concentration gradient of the gases isformed on the surface of the titanium; and (d) cooling the titanium. 2.The method of claim 1, wherein the atmospheric pressure of the step (a)is 5×10⁻⁵ torr or less.
 3. The method of claim 1, wherein the step (b)is performed for 10 minutes to 1 hour.
 4. The method of claim 1, whereina temperature in the step (c) is higher than that in the step (b). 5.The method of claim 1, wherein the step (c) is performed at 740 to 850°C.
 6. The method of claim 1, wherein the cooling of the titanium in thestep (d) comprises a step cooling method.
 7. The method of claim 1,wherein the step (d) comprises an aging step of maintaining titanium at500 to 800° C. for 30 minutes to 30 hours.
 8. The method of claim 1,further comprising after the step (d), a step of forming one or morecoating layers using over-layer coating methods such as a CVD method anda PVD method.
 9. The method of claim 1, wherein in step (c) the gasesare a mixture gas of oxygen and nitrogen, or a mixture gas of carbon andnitrogen.
 10. The method of claim 1, wherein the titanium is puretitanium or a titanium alloy.
 11. Titanium having a gradient-hardenedrigid layer formed by the method of claim
 1. 12. A titanium part havinga rigid layer formed by the method of claim 1.